1. Field of the Invention
The present invention relates to an image display apparatus, a control signal generating apparatus, an image display control method, and a computer program product. More specifically, the present invention relates to an image display apparatus, a control signal generating apparatus, an image display control method, and a computer program product, for controlling display in a liquid crystal display apparatus to perform AC (alternating current) drive.
2. Description of the Related Art
In a liquid crystal display (LCD), liquid crystal is sealed between two substrates provided with electrodes and a predetermined voltage is applied across the electrodes, so that orientation of the liquid crystal is changed and a light transmittance is controlled for performing display. However, if a unidirectional DC (direct current) voltage is applied for a long period, so-called burn-in occurs, where the orientation of liquid crystal molecules is fixed.
In order to overcome this problem, in display apparatuses using liquid crystal, such as a television set, a monitor, and a projector (hereinafter collectively referred to as an LCD), so-called AC drive of periodically switching a polarity of charge applied to the liquid crystal between [+] and [−] is performed to improve an afterimage characteristic and to prevent burn-in of the liquid crystal. A method of the AC drive is described below with reference to FIG. 1. FIG. 1 shows, in time series, pixels arranged in the vertical direction of a frame image that is displayed in a display unit 11. An input image is a 60 Hz image, and interframe spacing between respective times t1, t2, t3, and t4 is 1/60 sec.
In the method shown in FIG. 1, the polarity is switched between [+] and [−] for every vertical line (vertical direction in the figure) and every frame (time axis direction) in frame images of the respective times t1, t2, t3, and t4.
In another AC drive method, the polarity is switched between [+] and [−] for every pixel in a horizontal line of a frame, for every line, and for every frame. In any method, [+] and [−] alternately appear if a specific pixel is observed in a time direction. This is based on an assumption that “no DC component is accumulated if positive and negative charge polarities are alternately applied in time direction in a pixel when a typical normal image is displayed”. The burn-in can be prevented by the AC drive method. A known art of the AC drive method is disclosed in, for example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2003-36060).
The LCD performs display on the basis of a plane hold method, unlike in a CRT (cathode ray tube) using point sequential impulse drive. That is, when the LCD operates at a typical frame frequency of 60 Hz, the same image is held on the entire screen for a display period of one frame ( 1/60 sec=16.7 msec).
In the plane hold display, images are displayed by performing IP conversion, where an interlace signal is converted to a progressive signal. This is because many of content and broadcast signals applied to display of images are generated as image data according to an interlace method for a CRT.
The image data according to the interlace method is displayed in the following manner. Each image includes two fields. In a first field, a screen is scanned on every other horizontal scanning line from the top to lower end. Then, in a second field that has not been scanned, the screen is scanned on every other horizontal scanning line from the top. Accordingly, an image is displayed. When image content is displayed by the interlace method in a display apparatus performing plane hold display, such as an LCD, a line having a display image signal and a line not having a display image signal alternately appear in each display frame, so that flicker significantly occurs and the luminance decreases by half disadvantageously. In order to solve this problem, IP conversion of converting an interlace signal to a progressive signal is performed.
In the IP conversion, a signal of a line not having a signal included in an interlace signal is generated by interpolation. By applying the pseudo signal generated by the interpolation, the interlace signal is converted to a progressive signal, so that display is performed by using the progressive signal in which every pixel includes a signal. However, the progressive signal includes pixel data generated by the interpolation, which causes a problem that an image different from original content is displayed. In order to realize display of an interlace signal equivalent to the original content, interpolation pixels may not be displayed, that is, black pixels may be displayed. More specifically, as shown in FIG. 2, interpolation pixels generated by IP conversion are not displayed by decreasing the luminance level thereof, and only original pixels included in the interlace signal are displayed.
However, if such a display process is performed and if the AC drive described above with reference to FIG. 1 is performed, the following sequence is realized as shown in FIG. 2. That is, in a pixel 12, for example, an original pixel is displayed under a [+] voltage applied at time t1, a pixel having a luminance level of 0 is displayed under a [−] voltage applied at time t2, the original pixel is displayed under a [+] voltage applied at time t3, and the pixel having a luminance level of 0 is displayed under a [−] voltage applied at time t4. In the pixel having a luminance level of 0 displayed at times t2 and t4, the applied voltage is substantially 0. As a result, a [+] voltage is accumulated in a portion corresponding to the pixel 12 of the LCD apparatus, which causes burn-in. This is the same in the other pixel portions.